1. Field of the Invention
This invention relates to a process for the continuous hydrogenation of fats, fatty acids and fatty acid derivatives with hydrogen in the presence of a heterogeneous hydrogenation catalyst.
Processes for the hardening of unsaturated fats, fatty acids and fatty acid derivatives by catalytic hydrogenation are well known. Normally, the unsaturated compounds or mixtures thereof are directly reacted with hydrogen under a pressure of from 0.3 to 30 bar and at a temperature of from 50.degree. to 250.degree. C. in the presence of a heterogeneous hydrogenation catalyst comprising a noble metal, transition metal, or compound thereof, usually applied to a solid, insoluble support. The preferred transition metal is generally nickel.
The reaction velocity of the hydrogenation process is affected by a number of factors. One factor, which affects the reaction velocity at the beginning of the hydrogenation/hardening reaction, is the rate of transport of hydrogen from the gas phase across the phase boundary to the surface of the heterogeneous catalyst. At high hydrogenation temperatures, the rate of adsorption of the hydrogen onto the heterogeneous catalyst is also a crucial factor in determining reaction velocity. The adsorption of hydrogen onto the catalyst is based on an adsorption equilibrium which can be displaced towards better H.sub.2 adsorption by an increase in pressure, which is accompanied by an increase in the rate of the hydrogenation reaction. Also, the usual deactivation of the catalyst by catalyst poisons present in the reaction mixture may be at least partly compensated by an increase in pressure. For these reasons, optimal reaction temperatures and pressures are customarily experimentally determined for each catalyst and the associated substrate, and the reaction is thus carried out under these conditions.
2. Statement of Related Art
Continuous processes for the catalytic hydrogenation of fats and derivatives thereof in the presence of heterogeneous catalysts, in which the catalyst is dispersed in the liquid reaction mixture, are well-known in the art. In typical continuous processes, hydrogenation gas bubbles are flowed through the liquid substrate in a so-called "bubble-column reactor", or, alternatively, the liquid substrate is nozzle-injected into the gas space containing hydrogenating gas in a so-called "jet nozzle reactor". In these processes and also in continuous hydrogenation processes of the type which use a parallel-flow fixed-bed reactor (all of which are used on an industrial scale), as large a phase interface as possible is normally created between the liquid substrate phase and the solid catalyst phase to enable the hydrogenation gas to be uniformly distributed between the solid and the liquid phase. Maximization of phase interface for improved uniformity of distribution of the reaction gas between the solid phase and the liquid phase is commonly accomplished by an appropriate heterogeneous catalyst design, and by using gassing stirrers or similar gas-liquid circulation systems to promote distribution of reactants. However, the precise control of reaction temperature becomes difficult as the volume of the reaction vessel increases relative to the area of the vessel heatexchange surfaces. In addition, an isothermal temperature profile is generally developed for reasons associated with process technology. In batch operation, such as the non-continuous hydrogenation process carried out in autoclaves, in which the catalyst is dispersed in the liquid reaction mixture ("read-out") process, complete conversion of the liquid substrate to the hardened product is only usually possible with long reaction times. Unfortunately, in all these types of processes, satisfactory volume-time yields are rarely achieved. Back-mixing is a particular problem.
In order to overcome some of the disadvantages of the prior art processes, the reaction vessels employed in these processes, usually reaction tubes, have been furnished with a variety of packings to enlarge the contact surfaces available. Although these packings generally provide for uniform flow of the substrates and reaction gases which are passed through them, they also often impose on these substrates and gas a residence time which is too short to enable high conversions and high volume-time yields to be obtained. In one such process of particular interest, abstracted in Chemical Abstracts 87: 199 383b native oils are hydrogenated in steel tubes at 180.degree. to 220.degree. C. under a hydrogen pressure of from 2 to 50 atms in the presence of a nickel catalyst, employing glass Raschig rings to pack the reaction tube. This process gives good hydrogenation results when fats and oils having high iodine numbers, i.e., a high degree of unsaturation, are used. Poorer results, which render this process unsuited to general industrial application, are obtained when fats and oils having a low iodine number are used. In addition, the Raschig rings are necessarily irregularily arranged in the packed reaction tube, causing a comparatively large back-up of the substrate; this in turn gives rise to an irregular residence time of the substrate or the reaction gas in the reaction tube, which inevitably results in a reduction in the volumetime yield. This is a serious disadvantage in largescale applications. In addition, it has been repeatedly observed that in this process, the catalyst tends to disperse in the substrate sediments inside the Raschig rings and block flow through the rings, especially those rings of which the longitudinal axis is at a right-angle to the flow of the substrate or the reaction gas. The entire flow cross-section of the reaction tube is thus no longer available for the hydrogenation reaction, which results in a drastic reduction in the volume-time yields; also, this reduction in flow is accompanied by gradual blockage of the catalyst bed. In view of high volume-time yield requirements this process is economically unsuitable for use on an industrial scale.